Corneal Mycotic Infections
By William L. Miller, OD, PhD, FAAO
Although it may appear that interest in fungal keratitis has waned, cases continue to appear in our practices. It remains important to educate patients on compliance, which can be the first step in preventing microbial infection.
My first suggestion to patients who use multipurpose solutions is to practice active disinfection: rub and rinse lenses, report any redness and pain immediately and change lens cases often. If they must wear lenses while swimming or gardening, I recommend daily disposables. You may also suggest a prompt hydrogen peroxide disinfection after these activities, although a few new multipurpose disinfecting solutions (MPDS) show satisfactory log kill rates as well.
Fungal keratitis occurs most in warm, moist climates, and there are geographical differences related to the causative species. Initial diagnosis can be challenging because many patients can exhibit severe symptoms initially with little to no inflammatory signs.
Trauma with vegetative matter or other corneal traumatic events can provide the fungal species access to a normally intact corneal surface. Other associated risk factors include HIV, chronic ocular surface disease, herpes simplex keratitis and contact lens use (Ritterband et al, 2006).
Presentation and Diagnosis
Biomicroscopy often reveals a grayish-white infiltrative lesion with ill-defined borders. The excavated lesion—or in cases with yeast infections, elevated lesions—will stain with sodium fluorescein. Satellite lesions, often cited as a hallmark sign, may or may not be present. Many fungal keratitis cases mimic other infectious keratitis entities, such as bacterial, herpes simplex or acanthamoebic. Unless signs are undeniably pointing to fungal keratitis, the initial suspected infection should be treated empirically with antibiotics, such as fluoroquinolones, or more robust antibiotics, such as vancomycin in combination with gentamicin or tobramycin in severe, traumatic cases. Dose each antibiotic hourly and monitor patients within 24 hours.
In cases of unresolved or worsening infectious keratitis, explore other causes. For a fast, definitive assessment, instill proparacaine, take a corneal scraping or biopsy, and perform a cytotogical examination of the corneal specimen after a Gram, Giemsa or Periodic Acid Schiff (PAS) staining. You can directly plate the cultures, but this method may take several days to several weeks to demonstrate fungal growth. Both methods provide the proof necessary to initiate antifungal therapy.
In vivo confocal microscopy can also aid diagnosis but should not be the sole determinant in starting treatment.
Treatment typically starts with hourly administration of a 5% suspension of natamycin (Natacyn, Alcon). Natamycin is effective against filamentous fungi, such as Fusarium solani, but less effective against yeasts, such as the Candida species. Other antifungals include amphotericin B 0.15% and miconazole 1%.
Recent studies suggest that 1% voriconazole may be emerging as an ocular antifungal of choice (Tull, 2011; Siatiri et al, 2011). One advantage is that, unlike older antifungals, voriconazole may not require corneal debridement to increase penetration.
Topical therapy may continue for days or weeks with careful monitoring to assess the cornea's response to the medication. If the fungal keratitis is deep or has penetrated the cornea, systemic antifungals may be indicated; however, a prudent assessment of liver function before and during therapy is necessary.
Given the severe nature of the disease or treatment delay, some patients may require referral for surgical intervention. CLS
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Dr. Miller is an associate professor and chair of the Clinical Sciences Department at the University of Houston College of Optometry. He is a member of the American Academy of Optometry and the AOA, where he serves on its Journal Review Board. You can reach him at email@example.com